Try it with a high resolution video you've transcoded locally (so you're not dealing with streaming quality differences) at various resolutions. The human visual system is a lot less precise with video than it is with still images. A paused DVD frame often looks like a blocky, terrible mess, but it's perfectly fine when displayed as video.
I won't argue that if you pick your content carefully, and know exactly what to look for, you can tell the difference. But, practically speaking, most people watching a decently encoded video won't see a huge difference from 1080p to 4k at typical screen sizes and viewing distances, and will see even less difference going to 8k.
I honestly can't tell you if I'm watching 720p or 1080p content on our TV without looking at the source file info.
You're confusing "What can be perceived" with "What actually matters." I'd argue 4k was a bit of a stretch to sell new TVs, and 8k is going to be even more so.
We've hit, as someone else said, the 44kHz/16bit point of video. Encoding quality and bitrate matter a bit, just like mastering does with audio, but we're past the point of diminishing returns at this point.